Dual-clutch transmissions are known to consist of two partial transmissions, each with an input shaft that can be connected to a driving engine via an allocated separately actuable clutch. In the first partial transmission, for example, the gears for the odd gear ratios are disposed and in the second partial transmission are arranged the gears for the even gear ratios.
During operation, one gear ratio of a partial transmission is respectively active, when the clutch is engaged, while a gear ratio of the other partial transmission can be preselected, when the clutch is disengaged. In order to change gears, the engaged clutch is then disengaged and the disengaged clutch is engaged such that essentially no torque interruption occurs in the drive train. The engagement and disengagement of the two clutches, as well as shifting of the gear ratios, are carried out via a transmission control device. Electrohydraulic actuators are generally utilized as actuators for actuating the clutches, as well as for actuating the gear ratio levels. Special control measures must be met, if a malfunction occurs in the region of the transmission, for example, in one of the partial transmissions.
Described in DE 103 38 355 A1 is a dual-clutch transmission, in which the electrohydraulic control adjusts to an initial state in which both clutches are disengaged in case of a failure or malfunction of the electronic system which leads to an interruption of the drive torque in the drive train independent of the present driving situation. This is undesirable in at least some driving situations, since then the vehicle only coasts, when there is a failure of the electronic system, the driver is not warned ahead of time, and the vehicle cannot be purposefully parked.
In order to solve this problem, the above-mentioned DE 103 38 355 A1 proposes a control process in which the shifting states of the clutches, directly before failure of the clutch control or electronic system, should be retained at least in some shift states of the clutches in case of a malfunction in the region of the transmission. In order to achieve this, the dual-clutch transmission has a separate “state retention hydraulic.” In case of a failure of the clutch control or the electronic system, the state retention hydraulic controls first or second hydraulics which are allocated to the first or second clutch via hydraulic control lines in such a way at least in some starting shift states of the clutches, that the shift state that was present directly before the failure of the electronic system is retained. This allows at least completion of the present driving maneuver and purposeful parking of the vehicle.
Proposed in DE 10 2004 018 962 B3 is a process for controlling a hydraulic circuit of a dual-clutch transmission in which a safe state of the system is ensured by means of a safety circuit, when an error occurs. The safety circuit is designed in such a way, that, when a partial error occurs which is only allocated to one of the two clutches, this clutch is disengaged and, if a total failure occurs which is allocated to both clutches, the clutch whose clutch pressure was lower, when the total failure occurred, is disengaged and the other clutch whose clutch pressure was higher when the total failure occurred, is not disengaged, in order to continue transmission of torque via this clutch. If a partial failure occurs, the vehicle can continue being operated via the partial transmission that is not affected by the failure, so that limited driving operation is possible. If a total failure occurs, the presently existing driving state is retained, so that a driving maneuver can be completed and the vehicle can be safely parked.
In DE 103 38 355 A1 as well as also in DE 10 2004 018 962 B3, it is provided, that a presently applied control pressure is retained for at least one of the clutches by means of the separate state retention hydraulics or the separate safety circuit respectively, when a malfunction occurs which requires considerable hardware complexity in the hydraulic shifting system.